Field of the Invention
The present invention relates to a pneumatic tire provided with a tread pattern having land portions where sipes are formed.
Background Art
Conventionally, for example, slits or slots referred to as sipes are formed on land portions of blocks or ribs of a studless tire thus enabling stable traveling on an icy road surface having a low friction coefficient by making use of an edge effect generated by the sipes. As such a sipe, there has been known a so-called three-dimensional sipe where the shape of the sipe is changed in the depth direction. Linear sipes which open linearly on a tread surface or wavy sipes which open in a wavy form on a tread surface have been proposed in JP-A-2011-157011, JP-A-8-197915 and JP-A-2009-160986, for example.
However, when the number of sipes formed on the land portion is increased, the rigidity of the land portion is lowered so that the land portion falls excessively. Accordingly, a ground contact area is decreased and, at the same time, a ground contact pressure is locally increased so that ice on an icy road surface is melted whereby a water membrane is liable to be formed thus giving rise to a drawback that a braking performance of a tire on an icy road surface is lowered. Further, when the land portion falls excessively, the land portion is liable to be worn non-uniformly or the fallen land portion rubs a road surface when the fallen land portion returns to an original position after being separated from the road surface thus also giving rise to a drawback that an abnormal sound is liable to be generated.